Balancing method

ABSTRACT

A method of balancing a set of retaining and locking plates used to retain a row of gas turbine engine aerofoil blades in attachment to a rotor disc, includes: (i) providing a weighing plate having a circular track; (ii) providing a non-rotating, static balancing apparatus capable of detecting unbalance in the weighing plate; (iii) positioning the weighing plate on the static balancing apparatus, and positioning the set of retaining and locking plates on the track in an arrangement corresponding to a possible arrangement for the set of plates in use in the engine; and (iv) repositioning the retaining and locking plates on the track to arrive at a balanced arrangement for the set of plates.

This invention claims the benefit of UK Patent Application No.1308596.4, filed on 14 May 2013, which is hereby incorporated herein inits entirety.

FIELD OF THE INVENTION

The present invention relates to a method for balancing parts for a gasturbine engine, and more specifically to a method for balancing a set ofretaining and locking plates used to retain a row of gas turbine engineaerofoil blades in attachment to a rotor disc.

BACKGROUND OF THE INVENTION

Retaining plates are common to many gas turbine designs and are used toprevent axial movement of aerofoil blades such as turbine blades. Forexample, FIG. 1 shows a perspective view of gas turbine aerofoil blades1 attached to a rotor disc 7. The aerofoil blades 1 have root fixings(not shown) that are slidable into corresponding slots formed in thedisc 7. Retaining plates 3 are then used to retain the aerofoil blades 1in attachment to the disc 7. The retaining plates 3 themselves are heldin place with locking plates 5, which are smaller in circumferentialextent than the retaining plates, have a different mass per unit arc,and have a deformed profile for assembly reasons. The retaining andlocking plates may locate at one side in a grooved rim around the discand at the other side in a similar grooved rim in the inner platforms ofthe blades.

The retaining and locking plates are required to be balanced. Unbalanceoccurs when a mass centre of a rotor is different from its runningcentre axis. Units of unbalance are mass times radius. If a mass addedto a certain position on a part being balanced shifts the mass centreinto the running axis so that the part is in balance, the unbalance unitof the part is the mass of correction multiplied by the applied radius.The retaining and locking plates are thus balanced when the mass centreof the plates coincides with the running centre of the disc.

A conventional method for balancing a set of retaining/locking platesuses software to distribute the plates into a pattern of evendistribution based on the masses of the individual plates. If theunbalance of the complete set of plates is outside certain pre-definedlimits, the software swaps the positions of individual masses until thedesired result is achieved. Further balancing in a rotating balancingmachine may be required to reduce unbalance to within the pre-definedlimits.

A disadvantage of the software method is that it assumes that the platesare evenly circumferentially distributed. Almost invariably, however,this is not the case as the locking plates are usually a different sizeto the retaining plates. Further, the software method does not takeaccount of the typically different mass per unit arc of the lockingplates.

SUMMARY OF THE INVENTION

It would be desirable to have an alternative method for balancingretaining and locking plates.

Accordingly, a first aspect of the present invention provides a methodof balancing a set of retaining and locking plates used to retain a rowof gas turbine engine aerofoil blades in attachment to a rotor disc, theaerofoil blades having root fixings which are slidable intocorresponding slots formed in the disc, the retaining plates beingpositioned in a circumferential row at a rim of the disc to prevent theroot fixings sliding out of the slots, and the locking plates beingintroduced into the circumferential row between pairs of retainingplates to lock the retaining plates in position, the method including:

-   -   providing a weighing plate having a circular track corresponding        to the position of the circumferential row at the rim of the        disc;    -   providing a non-rotating, static balancing apparatus capable of        detecting unbalance in the weighing plate;    -   positioning the weighing plate on the static balancing        apparatus, and positioning the set of retaining and locking        plates on the track in an arrangement corresponding to a        possible arrangement for the set of plates in the        circumferential row at the rim of the disc; and    -   repositioning the retaining and locking plates on the track to        arrive at a balanced arrangement for the set of plates which        reduces unbalance produced by the set of plates to below a        predetermined threshold.

Advantageously, this method can compensate for angular differencesbetween plates, and facilitates their fast and easy balancing.

A second aspect of the invention provides a method of building a sectionof a gas turbine engine including:

-   -   performing the method of the first aspect;    -   providing a row of gas turbine engine aerofoil blades which are        attached to a rotor disc by sliding root fixings of the aerofoil        blades into corresponding slots formed in the disc; and    -   positioning the set of retaining and locking plates in a        circumferential row at a rim of the disc to prevent the root        fixings sliding out of the slots, the set of plates having the        balanced arrangement in the circumferential row.

Optional features of the invention will now be set out. These areapplicable singly or in any combination with any aspect of theinvention.

The circular track may be defined at one edge by a circular abutmentshoulder against which the retaining and locking plates can be located.Such a simple arrangement facilitates redistribution of the retainingand locking plates into a final, balanced, configuration for use in theturbine.

Conveniently, the method may further include measuring cold buildexpansion gaps formed between the retaining plates when positioned onthe track. The retaining plates can then be shortened, if necessary, toincrease the expansion gaps.

The retaining plates may have a different mass per unit arc than thelocking plates. The retaining plates may additionally or alternativelyhave a different circumferential extent than the locking plates.

The aerofoil blades may be turbine blades or compressor blades.

Other aspects of the invention provide devices, methods and systemswhich include and/or implement some or all of the actions describedherein. The illustrative aspects of the invention are designed to solveone or more of the problems herein described and/or one or more otherproblems not discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows a perspective view of gas turbine aerofoil blades attachedto a rotor disc and prevented from moving axially by retaining andlocking plates;

FIG. 2 shows a longitudinal cross-section through a ducted fan gasturbine engine;

FIG. 3 shows a plan view of retaining and locking plates located in acircular track of a weighing plate;

FIG. 4 shows a perspective view of the weighing plate of FIG. 3; and

FIG. 5 shows a cross-section view of the weighing plate of FIG. 3.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention, In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES OF THE INVENTION

With reference to FIG. 2, a ducted fan gas turbine engine is generallyindicated at 10 and has a principal and rotational axis X-X. The enginecomprises, in axial flow series, an air intake 11, a propulsive fan 12,an intermediate pressure compressor 13, a high-pressure compressor 14,combustion equipment 15, a high-pressure turbine 16, an intermediatepressure turbine 17, a low-pressure turbine 18 and a core engine exhaustnozzle 19. A nacelle 21 generally surrounds the engine 10 and definesthe intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

During operation, air entering the intake 11 is accelerated by the fan12 to produce two air flows: a first air flow A into the intermediatepressure compressor 13 and a second air flow B which passes through thebypass duct 22 to provide propulsive thrust. The intermediate pressurecompressor 13 compresses the air flow A directed into it beforedelivering that air to the high pressure compressor 14 where furthercompression takes place.

The compressed air exhausted from the high-pressure compressor 14 isdirected into the combustion equipment 15 where it is mixed with fueland the mixture combusted. The resultant hot combustion products thenexpand through, and thereby drive the high, intermediate andlow-pressure turbines 16, 17, 18 before being exhausted through thenozzle 19 to provide additional propulsive thrust. The high,intermediate and low-pressure turbines respectively drive the high andintermediate pressure compressors 14, 13 and the fan 12 by suitableinterconnecting shafts.

As discussed above in relation to FIG. 1, retaining 3 and locking 5plates are used to prevent axial movement of the aerofoil blades of e.g.the high, intermediate and low-pressure turbines 16, 17, 18. However,such plates can be used to retain other types of blades, such as theaerofoil blades of e.g. the intermediate and high-pressure compressors13, 14. The retaining plates 3 are positioned in a circumferential rowat a rim of the rotor disc 7 to prevent the root fixings of the bladessliding out of the slots formed in the disc, the radially inner edge ofeach retaining plate locating in a circular groove formed in the disc,and the radially outer edge of each retaining plate locating in acircular groove formed in the blade platforms. The locking plates 5 areintroduced into the circumferential row between pairs of retainingplates 3 to lock the retaining plates 3 in position, the inner and outeredges of each locking plate also locating in the aforementioned grooves.

FIG. 3 shows a weighing plate 109 suitable for use in balancing the setof retaining 3 and locking 5 plates, FIG. 4 shows a perspective view ofthe weighing plate 109, and FIG. 5 shows a cross-section view of theweighing plate 109. The weighing plate 109 is pancake-shaped and has acircular track 115 (best shown in FIG. 5) for receiving the plates. Thecircular track 115 corresponds to the position of the circumferentialrow of the set of plates 3, 5 in use at the rim of the disc 7, e.g. asillustrated in FIG. 1.

The circular track 115 replicates the running radius of the set ofplates 3, 5 so that the unbalance of the plates 3, 5 can be determined.The circular track 115 may be defined by a circular abutment shoulder113 which corresponds to the position of the circular groove in whichthe outer edges of the plates are located in use, as shown in FIGS. 4and 5. Conveniently, the plates 3, 5 can then be located against theabutment shoulder 113. This simple arrangement facilitates repositioningof the plates 3, 5 into different arrangements.

The weighing plate 109 is placed on a non-rotating static balancingmachine (e.g. a non-rotating vertical balancing machine available fromUniversal Balancing Ltd), preferably with the centre 111 of the weighingplate 109 (which corresponds to the axis of rotation of the rotor disc7) located centrally on the machine. The machine has instrumentationsuch that it can detect the unbalance of the plates 3, 5 with respect tothe centre 111 of the weighing plate 109.

Any unbalance in the set of plates 3, 5 located on the circular track115 is indicated by the balancing machine e.g. by way of a display. Theunbalance can then be corrected by manually repositioning the plates 3,5 around the track 115. The unbalance may be considered to be correctedand a balanced arrangement achieved when the unbalance falls below apredetermined threshold. The threshold can be set so that furtherbalancing in a rotating balancing machine is unnecessary. The thresholdis typically determined by the unbalance limits of the rotor, which area function of its mass properties and service speed.

To build the corresponding section of a gas turbine engine, aerofoilblades 1 are attached to a rotor disc 7 by sliding root fixings of theaerofoil blades into corresponding slots formed in the disc. The edgesof the retaining 3 and locking 5 plates are then located in theirrespective grooves in the balanced arrangement at the rim of the disc,to prevent the root fixings sliding out of the slots.

As illustrated in FIGS. 1 and 3, the locking plates 5 generally have asmaller circumferential extent than the retaining plates 3. Further thelocking plates 5 may have a different mass per unit arc. The angulardifference between the retaining plates 3 and the locking plates 5 isillustrated in FIG. 3, by the difference between angles A and B. Theapproach to balancing described above using the circular weighing plate109 and the non-rotating static balancing machine automaticallycompensates for this angular difference and for differences in the massper unit arc of the plates 3, 5.

The weighing plate 109 can also be used for checking that the retainingplate 3 have sufficient cold build expansion gaps therebetween. Ifnecessary, the circumferential lengths of one or more of the plates 3can be reduced by machining to increase their respective expansion gaps.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A method of balancing a set of retaining andlocking plates used to retain a row of gas turbine engine aerofoilblades in attachment to a rotor disc, the aerofoil blades having rootfixings which are slidable into corresponding slots formed in the disc,the retaining plates being positioned in a circumferential row at a rimof the disc to prevent the root fixings sliding out of the slots, andthe locking plates being introduced into the circumferential row betweenpairs of retaining plates to lock the retaining plates in position, themethod including: providing a weighing plate having a circular trackcorresponding to the position of the circumferential row at the rim ofthe disc; providing a non-rotating, static balancing apparatus capableof detecting unbalance in the weighing plate; positioning the weighingplate on the static balancing apparatus, and positioning the set ofretaining and locking plates on the track in an arrangementcorresponding to a possible arrangement for the set of plates in thecircumferential row at the rim of the disc; and repositioning theretaining and locking plates on the track to arrive at a balancedarrangement for the set of plates which reduces unbalance produced bythe set of plates to below a predetermined threshold.
 2. The method ofclaim 1, wherein the retaining plates have a different mass per unit arcthan the locking plates.
 3. The method of claim 1, wherein the retainingplates have a different circumferential extent than the locking plates.4. The method of claim 1, wherein the circular track is defined at oneedge by a circular abutment shoulder against which the retaining andlocking plates can be located.
 5. The method of claim 1, furtherincluding: measuring cold build expansion gaps formed between theretaining plates when positioned on the track.
 6. A method of building asection of a gas turbine engine including: performing the method of anyone of the previous claims; providing a row of gas turbine engineaerofoil blades which are attached to a rotor disc by sliding rootfixings of the aerofoil blades into corresponding slots formed in thedisc; and positioning the set of retaining and locking plates in acircumferential row at a rim of the disc to prevent the root fixingssliding out of the slots, the set of plates having the balancedarrangement in the circumferential row.